A combination of high-tech lasers and ground-penetrating radar have led to the rediscovery of 150-year-old military infrastructure buried below California’s notorious Alcatraz prison.

Before it was the world’s most infamous (and “inescapable”) penitentiary, the island of Alcatraz served as a military base from 1850 until 1933. Built into an island smack dab in the middle of some of San Francisco Bay’s most intimidating currents, the island is both within eyesight from the downtown hustle and bustle of the city while at the same time is isolated due to its shark-infested frigid waters.

When the prison was built at the turn of the 20th century, the former military base would not have been protected under cultural heritage and protection laws at the time, leading builders to “essentially bulldoze” the former military installation. Researchers wanted to see what – if any – of the fortifications were left following the transition.

“In converting the area to a prison, the vast majority of the previous military history of the island had been erased, but we wondered if perhaps something of that significant time in both the islands and American history remained, but buried and preserved beneath the subsurface,” explained archaeologist Timothy de Smet in a statement. “As such, we sought non-invasive, non-destructive means to ascertain if any historic archaeological remains lay beneath several parts of the island, like the Recreation Yard of the infamous US Federal penitentiary. We did not know what to expect. We did not know if there would be any extant subsurface architecture of these historically significant remains, or if there was anything left, what their extent and integrity would be like.”

An 1867 map of Alcatraz Island when it was a military fort. The defensive barracks shown in the center would later become the citadel and first prison. NPS/Wikimedia Commons

Using terrestrial laser scans, ground-penetrating radar data, and georectification (the process of taking old digitized maps and linking them to a coordinate system so that they can be accurately geolocated in 3D space), the team was able to find and analyze historical remains beneath what used to be the recreation yard for the prison. Writing in the journal Near Surface Geophysics, the team reports that remnants of old buried structures, “bombproof” earthwork traverses, vaulted brick masonry tunnel, and ventilation ducts remain in shockingly good condition.

“The remains of these historical archaeology features were just a few centimeters beneath the surface and they were miraculously and impeccably preserved. The concrete veneer of the Recreation Yard floor is incredibly thin and, in fact, in places sitting directly atop the architecture from the 1860s,” said de Smet. "We also learned that some of the earthwork traverses were covered over with thin concrete layers through time, likely to decrease erosion on the rainy windy island. It was wonderful to find the history just beneath our feet that we can visualize for the public.”

The researchers say this practice can be applied to other historical structures around the country, showing how modern technology can answer questions about human behavior without threatening the integrity of historical sites.

Today, Alcatraz is designated a National Historic Landmark and is open to the public for tours.

A hand-colored photograph from the 1930s shows the USS Oklahoma passing in front of Alcatraz. Courtesy of the USS Oklahoma Association, 1975. Collection of Irvin Barrett. Naval History and Heritage Command Photograph/Wikimedia Commons